1. Field of the Invention
The present invention relates generally to methods and systems for tracking the location of devices near the surface of the Earth, and more particularly to a method and system for tracking a device near the surface of the Earth using a very fast acquisition portable transponder.
2. Introduction
Various techniques are available to determine the position of a device on the surface of the Earth. U.S. Pat. No. 3,063,048 discloses the use of the Doppler shift in a transmitted signal to determine the position of a device transmitting the signal. This patent teaches measuring when the Doppler shift frequency is changing at a maximum rate to determine the position of the object transmitting the signal undergoing the Doppler frequency shift. In this system, the satellite must continuously receive the signal being transmitted from the object to determine when the frequency is changing at its maximum to locate the object. As a result, the time to compute a position fix is unacceptably long for applications such as locating a satellite telephone.
The Global Positioning System (GPS) also provides geo-location capability. Moreover, it is desirable to know the position of a mobile terminal, such as a cellular telephone, and to have this position information at a central location (e.g., at a Service Operations Center or SOC).
One technique is to place a GPS receiver on the mobile terminal, calculate position in the GPS receiver, and transmit via satellite (or other communications system) the position fix to a central location. This method suffers from the Time-To-First-Fix (TTFF) limit inherent in GPS receivers upon waking up from a cold start. From the cold start state, a GPS receiver must download the GPS satellite almanac, ephemeris, and clock correction information. The TTFF limit effectively eliminates using a GPS receiver in situations where a long TTFF is unacceptable. For example, in wireless or cellular telephone telephony applications, it would not be acceptable to require the user to wait for the mobile terminal (i.e., the wireless or cellular telephone) to download GPS data prior to making a telephone call.
The present invention is therefore directed to the problem of developing a system and method for calculating the position of a mobile terminal, which can be accomplished rapidly using a minimal amount of power and equipment.